The invention relates to a focal plane shutter, and more particularly to a focal plane shutter which is constructed to provide a time delay between the initiation of the winding of the first and the second screen thereof. More specifically, the invention relates to a focal plane shutter which provides a smooth shutter operation by an improved mounting of the screens on the winding or take-up drums, by controlling a uniform timing for the initiation of rotation of the winding drums when the winding operation is desired, or by controlling the operation of the winding drums.
In a conventional focal plane shutter which is used in a single lens reflex camera or advanced 35mm cameras, the winding drums associated with the first and the second screen are integrally provided with pinions, and a drive gear which is interlocked with a film winding operation is normally maintained in meshing engagement with both of the pinions associated with the respective winding drums. This results in a simultaneous winding of both screens upon charging the shutter, and if any slit is formed between the screens for some reason, there is no correction means therefor, resulting in both screens being wound while maintaining a slit therebetween to result in an exposure of the film, which is thus made useless. On the other hand, when the drive gear is normally in meshing engagement with both pinions, it is necessary to provide separately some means such as a one way clutch, for example, for disengaging the drive gear from the film winding mechanism during the shutter operation. This increases the overall size and the cost of the apparatus.
Several other considerations should be paid to the general arrangement of the focal plane shutter. Specifically, the first and the second screen of the shutter has its rear end secured to its winding drum, and unless the starting position of both screens is relatively adjusted in a proper manner, there may result a disturbance in a shutter speed during the shutter operation. When running the first and the second screen of the shutter, it is necessary to apply a smooth braking action toward the end of their running in order to avoid shocks which may occur at the termination of their running. On the other hand, it is desirable to avoid the application of the braking at the commencement of the running in order to assure a rapid actuation of the shutter, which may rather preferably be aided by application of a suitable amount of starting force. The provision of means which applies a braking or starting force in this manner usually requires a complex arrangement, which results in an increase in the cost.